The circadian timing system and the photoperiodic reproductive system are similar in that they are only responsive to light during the subjective night (SN). Light during the SN provides both systems with a phase shifting signal and gives an inductive signal to the reproductive systems of long day breeders. We have developed a new technique, feedback lighting (LD-FP), with which to investigate light effects. LD-FB links a behavior to the lighting condition in an environmental chamber, eg. nocturnal wheel running turns the chamber light on, thus selectively illuminating the SN. This application will use LD-FB to further examine light effects on the circadian and reproductive rhythms of hamsters. With respect to the circadian system, a prediction of the non-parametric model of circadian entrainment has already been verified with LD-FB, viz that light during the SN mimics the effects of constant light on the free-running period. We propose here to use LD-FB to repetitively illuminate specific portions of the SN and determine if the free-running period can be predicted by the non-parametric model. With respect to the photoperiodic system, we have shown that, contrary to the current understanding of photoperiodic regulation of reproduction, light during the SN, presented as LD-FB, failed to maintain gonadal activity. In this application we propose to investigate why LD-FB is not photoinductive. We will examine three possibilities: (1) LD-FB provides excessive LD transitions and is therefore disruptive (3) LD-FB disrupts reproductive rhythms by desynchronizing them or altering phase relationships.